One such servo system, employing a plurality of multiple disk clutches axially arranged in series to each other and a plurality of pistons arranged to be nested each other in the radial direction, has been disclosed in Japanese Patent Provisional Publication No. 2001-116060 (hereinafter is referred to as “JP2001-116060”). In the servo system disclosed in JP2001-116060, a fourth brake, serving as a start frictional-engagement element, and a third brake, serving as an engine braking means, are arranged in series to each other in the axial direction. A third-brake piston, used to engage and disengage the third brake, and a fourth-brake piston, used to engage and disengage the fourth brake, are arranged to be nested each other in the radial direction, so that the third-brake piston (the inside piston) axially slidably fits onto the inner periphery of the fourth-brake piston (the outside piston) to permit relative sliding motion of one of the pistons to the other.